When you share a common boundary, there are some obligations that both neighbours need to share when building a fence. The law states that if just one neighbour wants a boundary fence built where there wasn't one before, then the other neighbour has to share the costs.

However, some neighbours cannot afford to build a fence, or may refuse to do so. In this case you will have to take out a fencing order. The order is a legal document that covers fence height, materials, cost, and any other aspects of your neighbourly dispute.

If you do need to take out a fencing order you will have to check with your solicitor or your local courthouse. In New South Wales you can visit your local courthouse or the lands council to make an application.

If a fencing order is not completed, a court date will be set, so it’s in the best interests of both parties to settle any fencing issues before this occurs. If you have taken out the fencing order, it does not necessarily mean you will get everything you demand. Try and compromise with your neighbour as much as possible when it comes to cost, size and materials of a new fence.

Source: http://www.homeimprovementpages.com.au/article/what_is_a_fencing_order